1. Field Of the Invention
This invention relates to a powder weighing method, and more specifically it relates to a powder weighing method which makes possible high accuracy, wide-ranging and short-duration weighing by varying the speed of flow of the powder supply of a following cycle by means of a fuzzy inference performed on the basis of a weighing setting and the actual weighing value.
The present invention further relates to powder weighing mixers which produce new materials by mixing various kinds of powders after having weighed them.
2. Background Art
Conventionally, for weighing powders, there have been used scale systems which in the main make use of load cells.
There are known control systems which regulate the time taken and the flow rate in a following weighing cycle by calculating the average flow from the time taken and the overall discharge weight of a body being weighed in the prior prescribed count weighing cycles. These systems then find the deviation from the target weight based on this average flow amount have been disclosed in Japanese Patent (OPI) Publication Nos. 148019/81 and 155412/81. See also Japanese Patent (OPI) Publication No. 29114/82.
Thus, to date, there have been no closed loop weighing control methods which sequentially alter the speed of flow in accordance with the actual weighing value weighed in the receiving container.
Furthermore, when supplying powder from a plurality of supply containers (or tanks) to one receiving container in conventional powder weighing mixers, separate weighing equipment is attached to each of the supply containers.
For example, for heavy weighings two pieces of weighing equipment are used for two powders, as shown in FIG. 1. For closed loop control (one of the features of this invention) of these separate pieces of equipment, a two-loop control function is required for the pre-estimation control of the flow amount.
This is to say that it has not been possible to expect highly accurate weighings with a single control function because the powder flow varied as a result of the amount of powder remaining in the supply container, the target weight and the varying values of the physical properties of the powder.
Furthermore, there are weighing methods for realizing high accuracy weighing which change over to a slower speed of flow near the set target weight by providing equipment which possesses the capability of changing over to fixed conditions of differing speeds of flow as disclosed, for example, in Laid-open Japanese Patent Application (OPI) No. 72015/82. Alternatively, flow regulators of differing speeds of flow may be arranged in series. However, here too a two-loop control is required for the control function.
The reason for using the expression two-loop control function here is that when for example, dispersion-type control equipment is used, it is possible to compute the control functions with a single piece of control equipment so that two pieces of control equipment are, in fact, not required. However, it is still called two pieces of control equipment from the point of view of the software and the number of inputs and outputs.
Again, in connection with the above-mentioned methods, there are some methods which anticipate the amount of inflow to the weighing vessel at the cessation of weighing and stop the flow slightly in advance.
Because conventional weighing control methods have fixed weighing conditions within a prescribed range, either with a fixed speed of flow or dividing the speed of flow into two stages and changing over between them, as mentioned previously, they have the drawbacks mentioned below.
(1) Weighing accuracy: There are times when the accuracy cannot be guaranteed because of disturbances and changes in the physical properties of the powder.
Thus, the transfer equipment will differ with the physical properties of the powders. For example, dampers are used with granular powders because they have good flow characteristics and screw feeders are used with powders with poor flow characteristics. However, powder flow cannot be defined according to a single rule, and flow will vary with disturbances such as the consistency of the powder, the powder form and vibrations.
The flow characteristics of hygroscopic powders and powders which readily form bridges in particular will vary with their storage conditions. Thus, in a system in which powder is also stored over long periods in its supply vessels, the flow characteristics of a powder will change with changing environmental conditions, for example temperatures humidity and vibrations caused by the attachments such as vibrators, air knockers, etc., used to accelerate the flow characteristics of the powder. Thus, weighing accuracy declines with changes in the conditions of supply flow. There are, therefore, imposed limitations on the amounts stored and limitations on the installation conditions for the equipment with their resultant increase in the initial cost and running costs for component parts. These limitations are necessary to maintain weighing accuracy.
(2) The weighing range: The weighing range is narrow.
The reason for this is that there is an amount of residual inflow caused by delays in the response of the system even after the flow at one part of the system has been stopped. Since the amount is determined by the speed of flow, when the speed of flow is fixed, a tolerable amount of inflow can be guaranteed by narrowing the weighing range. Accordingly, even when weighing the same powder, if the weighing settings greatly diverge, weighing equipment suitable to each weighing range will be necessary and the number of units of equipment will increase.
(3) Weighing time: The weighing time is governed by the target weight.
The weighing time is short when the target weight is small and long when it is large. Weighing equipment with a weighing time appropriate to the production cycle is required in accordance with the target weight and the number of units of equipment is thus increased. Moreover, if target weights are to differ for each starting powder when several powders are to be mixed to produce a new mixed powder, for example, the production capacity of the system is determined by raw materials which require the longest duration weighings.
Furthermore, in conventional powder weighing mixers, many individually controlled units of weighing equipment are installed for each supply container for the above-mentioned reasons. Since they are installed for each optimal weighing time in order to increase the production capacity, the system is complicated and very many component parts are added to the weighing equipment.
Based on the above-mentioned facts, the present invention intends to provide a powder weighing mixer with the great economic advantages of:
(1) reducing the initial cost by reducing the number of units of equipment;
(2) reducing the labor spent on maintenance by similarly reducing the number of units of equipment;
(3) reducing breakdowns by improving reliability by reducing the number of units of equipment; and
(4) reducing running costs as a result of reducing raw material losses.
The weighing control equipment should not only produce high accuracy weighing unaffected by changes in the flow speed caused by disturbances and variations in the physical characteristics of the powder, but should also guarantee a wide range of weighing. It should effect weighing in a short time ungoverned by the size of the target weight. Thus, a system can be constructed which increases production capacity and simplifies component parts while reducing source material losses.